1. Field of the Invention
The present invention relates to a liquid crystal display element of a dot matrix system, and in particular to one excelling in flexibility.
2. Description of the Related Art
The rapid popularization of a type of electronic paper capable of retaining a display without a power source and enabling the electronic rewriting of the display content is predicted to happen in the near future. Research is in progress to find a type of electronic paper for accomplishing a reflective display that is gentle to the eyes so as not to tire them and that consumes an extremely low power amount, enabling a memory display to remain even after the power is cut off and enabling it to have a display body that is thin and flexible like paper. Conceivable applications of an electronic paper like this include, for example, electronic books, electronic newspapers, and electronic posters.
Electronic paper is categorized into an electrophoresis method, a twisted ball method, a liquid crystal display, an organic electroluminescence (EL) display, et cetera, depending on the difference of display method.
The electrophoresis method is a method for moving charged particles in the air or liquid. The twisted ball method is a method for rotating charged particles colored in two colors. The organic electroluminescence (EL) display is a self light-emitting display configured to sandwich a plurality of thin films made from an organic material by negative and positive electrodes. The liquid crystal display (LCD) is a non-self luminescent display configured to sandwich liquid crystal layers respectively by pixel electrodes and opposed electrodes.
Research and development of electronic paper made from an LCD is in progress, employing a selectively reflective cholesteric liquid crystal achieving bi-stability by utilizing an interferential reflection of a liquid crystal layer. Here, bi-stability means that a liquid crystal is stable in two different orientation states and a cholesteric liquid crystal is able to retain two stability states, i.e., planer and focal-conic states, for a long time even after the electric field is removed. In the cholesteric liquid crystal, an incident light is reflected in interference during the planer state and the incident light is transmitted during the focal-conic state. Therefore, a contrast of light can be displayed by using the selective reflection of an incident light on the liquid crystal layer in a liquid crystal panel using a cholesteric liquid crystal for the liquid crystal layer, thereby eliminating a necessity of a deflection plate. Note that the cholesteric liquid crystal is also called a chiral nematic liquid crystal.
A cholesteric liquid crystal that reflects a color via the interference of a liquid crystal is overwhelmingly advantageous in a color display of a liquid crystal display and therefore a color display is possible just by layering. Because of this, a liquid display system employing the cholesteric liquid crystal (which is called a “cholesteric liquid crystal system” herein for convenience) is overwhelmingly advantageous as compared to other systems, such as the electrophoresis method, in terms of color display. Other systems require a color filter divisionally colored in three colors for each pixel, causing the brightness to be one third that of the cholesteric liquid crystal. Because of this, the other systems are faced with a big obstacle in the improvement of the brightness for electronic paper.
As described above, while the cholesteric liquid crystal is a strong contender for electronic paper, the biggest problem has been the creation of physical flexibility that is a characteristic of electronic paper.
An LCD element requires a uniform cell with a gap of several micrometers, and a cell is formed by sandwiching a liquid crystal layer several micrometers thick where is between the top and bottom glass substrates. In some common liquid crystal panels of a twisted nematic (TN) type and a super twisted nematic (STN) type, an LCD element using a film substrate (i.e., a plastic liquid crystal) made from a transparent special resin has already been implemented. The plastic liquid crystal can be made thinner and of a lighter weight than a liquid crystal using a glass substrate, and further excels in durability and strength against bending. Therefore, the plastic liquid crystal is freely bendable like a paper and is accordingly suitable for electronic paper.
To accomplish a uniform cell gap in a liquid crystal panel of the TN or STN type, arraying support spacers 5 of a column form in the four corners of a pixel as shown in FIG. 1 has conventionally been proposed.
The liquid crystal panel of the TN type or STN type shown in FIG. 1 has arrayed the support spacers 5 in positions corresponding to grid points of a black matrix 6 on an upper surface substrate 2, wherein a liquid crystal layer is placed between the upper surface substrate 2 and a lower surface substrate 1 that are maintained at an equal distance from each other by the support spacers 5.
The support spacers 5 are featured on the lower surface substrate 1 on which, in addition to the support spacers 5, a seal member 3 for adhering between the upper surface substrate 2 and lower surface substrate 1 is featured. The seal member 3 is an adhesive member formed by printing or such, with the center of one side 3a being featured with an opening part, of which both ends are extended to form an injection hole 4. That is, the configuration is such that a part of the seal member 3 constitutes the injection hole 4 so as to inject a liquid crystal in the area enclosed by the seal member 3.
The top surface of the lower surface substrate 1 and the bottom surface of the upper surface substrate 2 are respectively featured with a plurality of transparent column electrodes (not shown in any drawing herein) and with a plurality of transparent row electrodes (not shown in any drawing herein), perpendicularly crossing the column electrodes. Also, the bottom surface of the upper surface substrate 2 is featured with a black matrix 6. In the LCD element configured, as described above, using a selectively reflective cholesteric liquid crystal in the liquid crystal layer, the part between pixels in which an electrode is not featured in the upper or lower opposed position is continuously lit. Consequently, the black matrix 6 is equipped for improving the contrast of a pixel by preventing it from being continuously lit. As for the method for forming circular or rectangular columns such as the support spacer 5, a forming method using photolithography has been proposed. Further, a liquid crystal display element in which the spacers are in a cross form have proposed.
In the case of a display panel of a selectively reflective cholesteric liquid crystal system, however, the mere accomplishment of a uniform cell gap cannot provide a material with flexibility. The reason for this is that the liquid crystal is a fluid so that the bending of a liquid crystal panel and/or the pressing of the display surface moves the liquid crystal forced by the added force of such action(s), resulting in a change in the display states. In the case of displaying a liquid crystal panel using the TN type or STN type, a change of display states can be reverted back to the original state because it is continuously driven electrically. In the cholesteric liquid crystal possessing a display memory property, however, the display cannot be reverted back to the original until it is re-driven.
An example of a method for featuring support spacers as shown in FIG. 1 for an element panel of the cholesteric liquid crystal is disclosed; a liquid crystal light modulation display element, however, mainly aims at securing uniformity in a cell gap therefore the liquid crystal light modulation display element is not configured to retain a memory property of a display element panel of a cholesteric liquid crystal system when performing operations such as bending a liquid crystal panel and/or pressing the display surface.
In the meantime, while the conventionally known capsule construction of a liquid crystal provides the effect of preventing a change in display states, it is faced with the problem of reduced contrast due to light diffusion noise from the capsule wall and that of increased drive voltage due to the capsule wall, thus resulting in a low possibility of implementing it as a commercial product. Particularly, such a light diffusion noise is a big problem for an LCD panel displaying color having a layered structure of liquid crystal layers for each of the colors red, green and blue (RGB).
With these backgrounds, the biggest problem has been implementing an LCD element having a structure that will not allow a change of display even if the electronic paper is pressed or bent in order to be able to apply a selectively reflective cholesteric liquid crystal to electronic paper.
The present applicant of the present invention has proposed an LCD element of the comprisal shown in FIGS. 2 through 4 as an LCD element of a cholesteric liquid crystal applicable to electronic paper.
FIG. 2 is a diagonal view diagram showing an overall three-dimensional configuration of the LCD element; FIG. 3 is a plain view diagram showing a positional relationship between support columns 15 and a matrix electrode for the LCD element; FIG. 4A is an overall layout pattern of the support columns 15 featured on the lower surface substrate 1; and FIG. 4B is a cross-sectional diagram of the column 15 in the horizontal direction.
The configuration is such that an adhesive support spacer 15 is formed to be a wall face structure body in approximately the form of a cross and gaps (i.e., opening parts) for injecting liquid crystal into the pixels are equipped between adjacent support spacers (i.e., columns) 15 as shown in FIG. 2. Also equipped is a wall face structure body 17 (which is called a wall face seal structure body 17 hereinafter for convenience) on the outer circumference of the surface of the lower surface substrate 1. The support column 15 and wall face seal structure body 17 may be made from the same material, making it possible to form them in the same photolithography process.
In the liquid crystal layer, a portion in which the column electrode (i.e., the signal electrode) 21 and row electrode (i.e., the scan electrode) 23 cross with each other constitutes a pixel 25, with the four corners thereof being equipped with support columns 15, respectively. A seal member 13 is placed on the outside of the seal structure body 17 with a prescribed distance apart therefrom. The necessity of the seal member 13 is arbitrary. While the bottom surface of the upper surface substrate 2 is featured with a black matrix 6, the plurality of support columns 15 is similarly patterned as the black matrix 6 with the layout position in the vertical direction of the pattern almost overlapping with that of the black matrix 6, and therefore it may be eliminated. Further, the support columns 15 are arrayed in the gap parts between the signal electrodes 21 and scan electrodes 23 so as to make the opening ratio of the pixels 25 be the maximum, as shown in FIG. 3.
The present applicant has proposed an LCD element having a configuration in which two kinds of support columns 15a and 15b are placed on the lower surface substrate 1 in the pattern as shown in FIG. 5A by virtue of the aforementioned PCT/JP/2005/4925. The LCD element is configured such that the support column 15a shown in FIG. 5 B and the support column 15b shown in FIG. 5C are placed alternately so as to make adjacent columns between the individual scan electrodes 23 different from each other.
Meanwhile, FIG. 6 is a diagram showing a plain view form of the wall face seal structure member 17 featured on the circumference of the lower surface substrate 1 (i.e., in the inside of the seal member 13) of the LCD element proposed. The wall face seal structure member 17 is of the same material as that of the support column 15 and is therefore formed simultaneously in the same process of forming the support column 15.
A prototype modeling of a cholesteric LCD element of the structure shown in FIGS. 2 through 4, however, has uncovered the problem with the LCD element as described in the following.
A fine and high-resolution display is desired for a display element, requiring a finer and smaller form of the electrode structure and wall face structure body (i.e., support columns 15). An electronic paper not using a light source such as a backlight is a reflective display element, strongly requiring improvement in an opening ratio for increasing the brightness (i.e., luminance) of the display.
In the aforementioned LCD display, a higher resolution was attempted by making the gap between electrodes be 10 to 30 micrometers and by narrowing down the width of the support columns 15 in association with the width of the gap, occurring the problem of the support column 15 coming off in the development process of the photolithography process.
The LCD element of the electronic paper commonly uses a plastic film for the substrate. When this is the case, aplastic surface of the substrate is exposed in the gap part between the electrodes on the substrate. The electrode uses a transparent electrode material such as indium tin oxide (ITO). The wall face structure body placed in the gap part between the electrodes has a low degree of adhesion to the plastic surface. A high-resolution pattern of the wall face structure body where the adhesion area size with the substrate 1 becomes small has a structure allowing easy detachment from the plastic surface. Because of this, it can seem that detachment of the support column 15 has occurred. Furthermore, observation of the phenomenon of the detachment shows that the beginning of the detachment is at the tip part of the branch part of the support column 15 of the cross form.
Meanwhile, the wall face seal structure body 17 as shown in FIG. 6 has conventionally been formed to be a few millimeters thick, in consideration of durability. In order to achieve a high-resolution LCD element, such a pattern must be formed finer and smaller, i.e., at a higher resolution. If the pattern is formed at a higher resolution, however, detachment will occur more remarkably in the development process of a photolithography process for forming the support columns 15. This is attributable to a decreased adhesiveness occurring as the seal structure body 17 of a line form becomes more slender.
The material of the wall face seal structure body 17 exerts the function of making the upper and lower surface substrates adhere with each other as a result of a pressurized heat treatment being applied. A reaction gas is generated in the process that the material becomes adhesive and hardened. Further, it has also been discovered that a large number of air bubbles remain in the adhesion surface between the lower surface substrate 1 and wall face seal structure member 17 due to the gas, causing an adhesion failure, which is a factor of the detachment. Furthermore, the remaining gas flows into the liquid crystal layer of the display part, and thus the remaining gas causes an occurrence of a display failure.